The present invention relates to a process and a device for co-injection molding multi-layer elements and to the products thus obtained.
In particular, the invention relates to the production of elements provided with at least one part having an external skin which covers an internal core material. Examples of such product are trim panels and trim elements of motor vehicles.
To obtain products of a type similar to those described above co-injection processes are already known which provide for the sequential injection of two or more materials from a single injector nozzle. In these processes a preset amount of skin material is initially injected into the mould, followed by one injection of core material and then another injection of skin material.
The European Patent No.0579925, in the name of the applicant, describes a device for co-injection molding provided with independently heated hot channels and nozzles located in the wall of the mould. The nozzles described may be either coaxial or side-by-side and they allow two or more materials to be injected successively into the same mould cavity from a plurality of points. PCT application No.WO 95/17291, in the name of the applicant, describes a process for co-injection of products with complex shapes. According to this process, the thickness of the mould is controlled according to the rheological properties of the material to be injected in order to obtain flow paths having substantially the same resistance and thus achieve homogenous distribution of the material inside the mould. The materials are injected via nozzles of the type for sequential injection of two materials; in the preferred embodiment only one nozzle is used for a mould for a car panel.
These processes allow products with complex shapes, such as car panels with integral compartments, handles and grilles for loudspeakers, to be produced with a uniform external skin and an internal structural core. However, these processes require high injection pressures, which in turn mean large presses, flash on the molding, materials that are particularly fluid, or a greater thickness of moulds, with their related costs.
It is an aim of the invention to solve the aforementioned problems and to provide a simple and economical process for co-injection molding of two or more materials to give products having at least in part an external skin and an internal core.
A further aim of the invention is to provide a device to carry out the above described process.
Such aims are achieved by the present invention which relates to a process for co-injection molding of multi-layer products using of two or more materials, comprising a plurality of nozzles located in a corresponding plurality of distinct points on the same cavity of a mold characterized by at least one of the said nozzles being provided with means of feeding only one of the said materials. According to the preferred aspect of the invention, only one material is injected at each point of the injection and the materials are preferably cascade injected.
The invention further relates to a device for co-injection molding multi-layer products as set forth above.
A further object of the invention are the multi-layer products obtained by the process of the invention, which comprise at least one portion provided wit one external skin layer and one or more internal core layers The process is characterized by having a plurality of points of injection, at least one of the said points being dedicated to only one of the said materials.
The invention has several advantages over known processes. Lower injection pressures are required, therefore smaller and less expensive presses can be used. Also, the molding-cycle time is shorter than in prior art. Furthermore, junction lines between the flows of materials are avoided and the mould can have substantially constant thickness for the greater part of its extent, if this does not conflict with the design requirements. The constant thickness results in greater facility of design and less weight in the finished product.
Another advantage lies in being able to produce the fixing elements to assemble the panels onto the doors and the counter-panels or complementary panels (retainers and studs) out of core material alone, i.e. with a rigid material. Thus, very soft materials can be chosen for the skin layer, improving the xe2x80x98feelxe2x80x99 of the finished product.